Microelectronic packages called "multi chip modules" (MCMs) are constructed with unpackaged semiconductor dice. With a multi chip module, a number of dice are attached to a printed circuit board or other substrate and electrically connected to form various circuits and electronic devices.
One reason for the increased use of multi chip modules is increased system performance. In particular integrated circuits on multi chip modules can be operated with lower resistance and parasitic capacitances. This is largely a result of decreasing the interconnection length between the dice included in the multi chip module. In addition overall system performance is improved because the input/output ports can be configured to access the whole module, which can be organized to reduce signal delays and access times. The power requirements are also reduced due to a reduction in the driver requirements.
Typically the dice are mounted on a substrate having an interconnect pattern formed using a process such as screen printing. Different techniques are used for mounting the dice to the substrate and for providing interconnection and termination of the unpackaged dice. These techniques include wirebonding, tape automated bonding (TAB), micro-bump bonding and flip chip bonding.
With flip chip bonding, each die is mounted circuit side down, and bond pads on the die are bonded to corresponding connection points on the substrate. Flip chips are formed similarly to conventional dice but require an additional process step to form solder bumps on the bond pads. The solder bumps are typically formed with a height of 25 .mu.m to 75 .mu.m. The solder bumps separate the dice from the substrate and minimize the actual contact between the dice and substrate.
One important consideration in constructing multi chip modules is the electrical connection between the bond pads of the unpackaged dice and the connection points on the substrate. It is important that these electrical connections provide a low resistance or ohmic contact. Additionally, it is preferable that each electrical connection be established with a minimum amount of trauma to the dice and particularly to the bond pads of the dice. The integrated circuits within a die can also be adversely affected if heat or thermal cycling is used to make an electrical connection.
Another important consideration in fabricating multi chip modules is the effect of thermal expansion on the electrical connections. If the dice and substrate expand by a different amount, stress may develop at the connection points and adversely effect the electrical connections. Stresses from thermal expansion can also lead to damage of the dice and substrate. For this reason silicon substrates are often used to construct multi chip modules.
Another problem in the manufacture of multi chip modules is that the size of semiconductor dice and the size and spacing of the bond pads on the dice have become smaller. This makes mounting and interconnecting of the dice on a substrate more difficult. In some electronic devices, such as computers, it is often necessary to integrate a large number of dice into an assembly to provide an extended memory or other component. This compounds the problems outlined above. The present invention recognizes that it is advantageous to construct multi chip modules using fabrication techniques employed in semiconductor manufacture.